Add plasma neutron surface power

[chris-ashe]

This pull request introduces a new physics variable to track the average neutron flux at the plasma surface and integrates it throughout the codebase. The changes ensure this value is calculated, initialized, displayed in plots, and outputted for analysis. Here are the most important updates, grouped by theme:

Addition and Initialization of New Variable:

• Added plfux_plasma_surface_neutron_avg_mw to physics_variables.py and initialized it in init_physics_variables() to enable tracking of the average neutron flux at the plasma surface.
  Calculation Logic:

• Computed plfux_plasma_surface_neutron_avg_mw in the physics() method by dividing total neutron power by plasma surface area, ensuring accurate physics modeling.

Output and Visualization:

• Added plfux_plasma_surface_neutron_avg_mw to the output in outplas() for reporting and analysis.
• Updated plot_main_plasma_information() to display the new neutron flux value in plasma information plots, improving data visualization

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.47%. Comparing base (ca03a41) to head (43e060e).

Files with missing lines	Patch %	Lines
process/physics.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4087   +/-   ##
=======================================
  Coverage   46.47%   46.47%           
=======================================
  Files         123      123           
  Lines       28902    28907    +5     
=======================================
+ Hits        13432    13435    +3     
- Misses      15470    15472    +2     

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[j-a-foster]

Happy with changes.

[timothy-nunn]

Happy with code changes, JF happy too